str-slice removes quotes

[PhilippSoehnlein]

input.scss

$foo: "'abcd'"; // '"abcd"' also doesn't work

@error str-slice( $foo, 1, 2 ); // not real code, @error is just used to get raw value of $foo

Actual results

libsass 3.5.4

ab on line 3 at column 1

Expected result

'a on line 3 at column 1

ruby sass 3.5.6

'a on line 3 at column 1

version info:

$ node-sass --version
node-sass	4.10.0	(Wrapper)	[JavaScript]
libsass  	3.5.4	(Sass Compiler)	[C/C++]

Further context

We have a string-utility function in our code base, which slices bigger strings in smaller pieces using str-slice. It works fine, except when the piece that should be sliced out of a bigger string coincidentally starts with quotes and ends with the same quotes, then str-slice returns the sliced string without the quotes, which makes our function return incorrect stuff.